Marshall AVT20 no sound

[Jazz P Bass]

"still no sound through the guitar input but it’s louder through the CD input:

The CD input goes directly to IC4 pin 6& then to the output ic, TDA2050.
So if that works and the Guitar input does not then your problem lies between the input jack and IC4 pin 6.

You need to trace the signal through the preamp to see where it drops out.

[Mick Bailey]

The original fault was he could hear a faint guitar and faint output from the CD input (everything on 10). My thought is to get the CD input working properly at least to establish that the signal path from IC4b onwards is working. As it's a more-or-less direct connection between the CD input and IC4b it should be easier to trace. If I recall the CD input on these amps gets a lot louder than the guitar input when the CD source level is turned up.

[Jon Snell]

Follow the signal through the signal path, you will soon find the error.
I cannot post a schematic as it is copyrighted.
Check R3.

[g1]

Schematic was provided in post #4.

[bfglp]

Follow the signal through the signal path, you will soon find the error.
I cannot post a schematic as it is copyrighted.
Check R3.

I have the schematic thanks for your input. Trace and test is my next step

[Jon Snell]

Go back to basics.
Input a signal to the TDA2050 pin 1, do you get output?
If you do, forget all about the output stage.
Input a signal on the top (pin 1) po the volume control. Do you get a signal? If you do forget all about the driver stages as well.
Is R* and R10 OK? Have you got HT on one side and Anode voltage on the other?
Input a signal on pin 6 IC3 etc etc etc

See where I am going with this?

[Mick Bailey]

He gets some output from the CD input, and originally said the guitar input was faint. The question is, how loud is the CD output compared to what it would normally be? If the output IC was dead there would be no sound at all.

Could the OP confirm this, and is there still a faint guitar output with everything on 10? Did you check the +/- 15v?

[bfglp]

Could the OP confirm this, and is there still a faint guitar output with everything on 10? Did you check the +/- 15v?

plugged in a guitar into instrument input jack everything on 10 clean channel faint sound a bit louder with OD engaged. Plugged guitar into CD channel and it’s twice as loud but still only like a healthy AVT20 with volume on 1

IC4b checks out ok

[Jon Snell]

I will try once more.
If you cannot complete any of these basic checks, take it to a tech who can.

Have you checked the HT, the Anode voltages and the signal through from the input socket to the Volume control? HT should be 130volts ish. The anodes around 90volts with no signal.
That is the only easy way to find the fault.

The guitar into the CD input.
Guitar gives about 500mV of signal. CD gives almost 10 times that. That is probably why it is quiet through the CD input.

[bfglp]

I will try once more.
If you cannot complete any of these basic checks, take it to a tech who can.

Have you checked the HT, the Anode voltages and the signal through from the input socket to the Volume control? HT should be 130volts ish. The anodes around 90volts with no signal.
That is the only easy way to find the fault.

Anode’s are @ 82.6v and HT is @ 127v

i have yet to signal trace It’s on the to do list

voltage on the IC’s are all acceptable

I appreciate everyone’s input and help keeping my investment to a minimum. It’s been a great practice amp for the $40 purchase I made a few years ago.

[Mick Bailey]

I assumed you were using a CD for the CD input. If you do (or an MP3 player or phone output) is the amp loud? This will tell you that IC4b and the TDA are working OK.

[bfglp]

I assumed you were using a CD for the CD input. If you do (or an MP3 player or phone output) is the amp loud? This will tell you that IC4b and the TDA are working OK.

With phone @ full volume plugged in CD input it’s at an except able volume. I never used this feature it’s no where near as loud as playing my guitar through a healthy AVT20